15.3. THEPERIODICTABLE 249
4 s-orbitaltendstobeclosertothenucleus,andhenceseesalargereffectivepositive
charge,thanthe 3 d-orbital,whichtendstobefurtherfromthenucleus,whosecharge
isthereforescreenedtoagreaterextentbyinterveningelectrons.
KnowledgeoftheatomicnumberZofanatom,andtheorderofenergies(15.69),
allowsustoimmediatelywritedowntheconfigurationofelectronsinthegroundstate.
Asanexample,considersodium,withanatomicnumberofZ=11. Oftheeleven
electrons,thefirsttwogointothe 1 sstate,whichisfilled. Twomoreelectronsgo
intothe 2 sstate,whichisalsofilled.Oftheremainingsevenelectrons,sixfillthe 2 p
level,andtheremainingelectronoccupiesthe3slevel.Sotheelectronconfiguration
iswritten^4
Na: 1 s^22 s^22 p^63 s (15.70)
Electronsinthehighestenergylevelareknownasvalenceelectrons. Itisthese
electronsthatdeterminethechemicalpropertiesoftheatom. Sodiumhasonevalence
electron,inthe3sstate.
- A Quick Tour of the Periodic Table: The Periodic Tablebegins with
Hydrogen,witha 1 selectronconfigurationinthegroundstate,andan ionization
enegyof 13. 6 eV.Thenextelement,Helium,hastwoelectronsina 1 s^2 configuration,
but(asweseeinFig. [15.3]),theionizationenergyhasalmostdoubled,to 24. 6 eV.
Thereasonforthisincreaseinionizationenergyisquitesimple: thechargeofthe
nucleusistwiceasbigasforHydrogen,soeachofthe1slevelsismoretightlyboundto
thenucleus. Heliumwillnoteasilygiveupanelectrontoformamolecule;moreover,
sincethe 1s level isfilled, itwillnot capturean electron either. Thiselementis
thereforechemicallyinert.
Theelectronconfigurationofthethirdelement,Lithium,is 1 s^22 s,andtheion-
izationenergyismuchlessthanHelium,beingonly 5. 4 eV.Naively,sincethecharge
ofthenucleusisnowZ=3,onemightexpecttheionizationenergytoincrease,as
itdidwithHelium,sothatLithiumwouldbeevenmoreinactive. Itisatthispoint
thattheExclusionPrinciplecomesintoplay,withaconstraintthatallowschemistry
tobeinteresting,andlifetoexist.Sincethe 1 slevelisfilled,thethirdelectronmust
occupy the 2 slevel. But anelectroninthe 2 slevel isfurtheraway fromthenu-
cleus,onaverage,thanthetwoelectronsinthe 1 slevel,sothevalenceelectronsees
ashieldedcharge,whichisonlyaboutZ=1.Havingseenthis,wecanestimatethat
thebindingenergyofthe 2 selectroninLithiumshouldbeaboutthesameasthatofa
2 selectroninHydrogen,namely 3. 4 eV.Theactualbindingenergy(whichisalsothe
ionizationenergy)isinfactslightlyhigherthanthat(5. 4 eV,asmentionedabove),
andthisisbecausetheelectroninthe2sstatehassomeamplitudeforbeingclose
tothenucleus,whereitwouldseealargereffectivenuclearcharge. Movingtothe
nextelementinline,Beryllium,theconfigurationis 1 s^22 s^2 ,andagaintheionization
(^4) Whenthereisonlyoneelectroninagivenlevel,oneomitsthesuperscript,i.e. 3 sā” 3 s (^1).